GO curators for mouse genes have assigned the following annotations to the gene product of Btk. (This text reflects annotations as of Tuesday, May 26, 2015.) Summary from NCBI RefSeq

[Summary is not available for the mouse gene. This summary is for the human ortholog.] The protein encoded by this gene plays a crucial role in B-cell development. Mutations in this gene cause X-linked agammaglobulinemia type 1, which is an immunodeficiency characterized by the failure to produce mature B lymphocytes, and associated with a failure of Ig heavy chain rearrangement. Alternative splicing results in multiple transcript variants encoding different isoforms. [provided by RefSeq, Dec 2013]Summary text based on GO annotations supported by experimental evidence in mouse

Researchers have inferred from direct assay, that the gene product of Btk